This paper describes how the Oxylog, an apparatus for measuring the respiratory ventilation volume and oxygen consumption of ambulatory human brings which is manufactured by Morgan, Kent, UK has been adapted for use with draught animals up to 500 kg live weight. Modifications included a suitable face mask which can be made cheaply and easily for animals of different sizes and two types of enlarged flow meters allowing ventilation volume (VV) up to 320 l min(-1) and oxygen consumption (VO2) up to 13 l min(-1) to be measured. When tested against a calibrated piston pump, the modified Oxylog performed as well as the original and measured VV and VO2 to +/-1%. Determinations of VO2 of a cow agreed to within +/-6% with simultaneous measurements obtained from an open circuit, laboratory based respirometric system (average difference 1.5% SD+/-3.6, n = 14). Examples are given of results obtained from animals working in the field in Nigeria and Nepal.